Can You Recruit the Sample Your Study Needs?

Start with two different numbers

A required analyzable sample and the number you need to enroll are different quantities. A justified sample-size calculation does not establish that your clinic can recruit that many eligible, consenting participants in the available time.

Write down the source of your expected recruitment rate: a screening log, a comparable local study, or an explicit assumption awaiting verification. A clinic attendance count is not an enrollment rate. Check repeat visits, eligibility, competing studies, and the capacity to obtain consent.

Work through a fictional scenario

Suppose a study needs 200 participants with the planned outcome measured. With an assumed 10% loss, the enrollment target is ceiling(200 / 0.90) = 223. At 20 enrollments per month across all sites, that implies 12 months of recruitment, rounded up. These are hypothetical inputs, not observed study results.

The last participant still needs follow-up. If the outcome is measured six months after enrollment, recruitment duration alone is not the date when the final outcome becomes available. Add approvals, setup, site activation, and data checks separately.

Test the assumptions before committing

Use the Recruitment Timeline Planner to compare a base scenario with slower enrollment or higher loss. Keep the scenarios labeled; do not silently replace the original assumptions after seeing the results.

Assign an owner and a review date to each uncertain input. During recruitment, compare actual screening, eligibility, consent, and retention with the plan. Investigate a shortfall before changing eligibility or outcomes, and follow the applicable amendment process.

What this does not solve

Inflating enrollment does not correct selection bias or bias from missing outcomes. Different groups may require separate planning, and a clustered design needs design-specific sample-size methods. The target should come from the actual scientific question and analysis plan, not from the convenient number of available patients.

Download the recruitment assumptions worksheet. For missing-outcome concerns, see Cochrane’s discussion of missing outcome data.